1. Field of the Invention
The present invention relates to a portable, hand-held and nondestructive moisture-sensing device, based on electrochemical impedance spectroscopy (EIS), for detecting moisture in graphite/epoxy, glass/polyamide, and other composites and adhesively bonded structures. Repair operations in composite laminations or honeycomb structures become difficult and time-consuming in the presence of moisture, even at moisture contents as low as 0.5%. If the moisture is not removed by slow heating or exposure to a dry atmosphere, steam formed at elevated curing temperatures can result in more extensive damage than existed before the repair attempt. Moisture in honeycomb structures can result in corrosion of the thin core material and loss of structural integrity; moisture in an adhesive can reduce the mechanical strength of the adhesive and reduce interfacial bonding, and allow corrosion on hydration of the substrate at the interface with the adhesive. Under these conditions, an in-situ corrosion sensor was adapted to detect moisture in composites and honeycomb or adhesively bonded structures, even at moisture contents as low as 0.5%. It detects moisture in graphite/epoxy/Nomex honeycomb structures and aluminum/epoxy/aluminum structures is applicable to other bonded structures, as well. Coupled with a commercial portable computer, the hand-held sensor is capable of detecting moisture in composite and bonded components of various structures, including, for example, the F/A-18, F-14, EA-6B, and V-22 aircraft and the Boeing 777 series, among others.
In particular, the present invention relates to a portable or permanently fixed moisture sensor which is utilized under field (actual, environmental or in situ) conditions in detecting moisture in composite laminations or honeycomb structures, as well as other bonded structures, thereby preventing the formation of steam, resulting in subsequent structural voids, delaminations and disbonds during repair operations. It can also warn of possible moisture-induced degradation resulting from hydration of aluminum bonding surfaces or corrosion of core material well before serious deterioration of the material or structure has occurred.
2. Prior Art
A major goal in the electrochemical field has long been to create a sensor which could be utilized in field or service conditions to detect moisture of composite laminations or honeycomb and adhesively bonded structures of any size, before significant degradation has occurred. Moisture concentrations less than 1% in the composite or in the adhesive between the composite and the core can cause significant structural damage. To compensate for the lack of real-time data, the prior art relied on the drying schedules for parts requiring repair to be conservatively set, in order to prevent the possibility of entrapped moisture, particularly in large parts. Drying temperature and time are based primarily on historical experience and success for each material system and component configuration, with a typical schedule of several days at 160-180.degree. F. (ambient pressure). The general consensus has been that larger composite parts presented a greater risk, because it was difficult to ascertain adequate drying schedules or to prevent entrapped moisture.
The only prior art available to inspect high-performance moisture content of composites, such as those used in the aerospace industry, is a method which teaches monitoring of the humidity level of the drying air, much like the humidistat in an household clothes dryer. Other prior art technologies that have been utilized include thermography, x-ray and neutron radiography. Thermography appears useful in detecting, but not quantifying trapped moisture in honeycomb materials. X-ray and neutron radiography are both quite sensitive in detecting trapped moisture, but their safety restrictions, ease of use, and costs have been major deterrents to date. Furthermore, portability has been a previous disadvantage to these radioscopic techniques.
Other moisture sensors are currently commercially available. These are either grossly qualitative instruments or expensive laboratory contraptions not suited for the target application. Of those that have relevance, the principles of operation of the meters themselves tend to fall into 3 categories: conductance/capacitance, microwave and infrared absorption. Most commercial moisture meters seem to be directed at either the boating, wood products or the food (i.e., grain, cereal, etc.) industries and not at the high-performance, typically conductive composites or adhesive bonds.
Conductance and capacitance meters are used primarily by the boating industry. The pin-type detector measures dc resistance between two probes pressed into the composite. Because of the damage generated by the pins, this type of detector is generally limited to wood and masonry. The pad-type detector transmits and receives radio frequency signals and relies on water's higher dielectric constant compared to the glass reinforced plastic or air. It is not suited for graphite composites due to the electrical conductivity of the fibers and it is limited in the depth of material that it can inspect.
Microwave dielectric-based systems and infrared meters are research lab instruments not suited for field applications. The infrared systems require a small physical sample for evaluation.
By contrast to the prior art, the proposed moisture sensor is small, rugged, suitable for conductive and nonconductive composites and honeycomb and adhesively bonded structures, and does not require insertion into the structure or cause other damage.
Presently, there is no portable, hand-held or permanently attached, in situ and moisture-detecting and monitoring device for high-performance composite laminations, honeycomb or other bonded structures which can evaluate degradation on structures or material of any size, under in situ or actual conditions, as well as under aggressive corrosive conditions, and requires no permanent attachment.